Mastering the scientific use of compound fertilizer technology starts with understanding its characteristics, as well as the nutritional needs of crops throughout their growth cycle and the basic properties of the soil—such as nutrient release rates, moisture levels, and fertilizer retention capacity. While this may sound complex to some farmers, experienced growers take these factors into full consideration in real-world applications.
Compound fertilizers are produced through secondary processing, typically by mixing powdered raw materials like urea, calcium sulfate, and potassium chloride, along with a small amount of binder, and then granulating them. These fertilizers come in granular form, containing nitrogen, phosphorus, and potassium, and may also include trace elements such as calcium, magnesium, sulfur, iron, or molybdenum. The nutrients in compound fertilizers can be fully water-soluble, partially insoluble, or only weakly soluble in acid. For example, some phosphate compounds are both water-soluble and weakly acid-soluble. Therefore, the main features of compound fertilizers include: first, granules with certain compressive strength that gradually release nutrients when applied to the soil; second, a complex composition that includes multiple macronutrients and sometimes micronutrients; third, different forms of nutrients—like nitrogen in the form of urea, ammonium, or nitrate; phosphorus that can be fully or partially soluble; and potassium that may be sulfate- or chloride-based. In short, compound fertilizers, after secondary processing, have higher technical value, which increases production costs and prices. Therefore, it's essential to apply them correctly based on crop and soil conditions for optimal yield and profitability.
Based on these principles, the proper application methods of compound fertilizers can be summarized as follows:
1. Compound fertilizers are best used as base fertilizers. When preparing the soil before planting, they are applied at a certain depth to supply nutrients throughout the growing season. The depth varies depending on the crop—typically 20 cm for grains and vegetables, and 30–40 cm for fruit trees. Applying compound fertilizer at the right depth ensures long-term nutrient availability. Most of the phosphorus and potassium needed during the growth period is provided through base fertilizer, while nitrogen should be supplemented later with simple nitrogen fertilizers, as it is more volatile. Thus, the nitrogen content in compound fertilizers should not be too high.
2. The ratio of N-P-K in any compound fertilizer is fixed, so the base fertilizer is usually calculated based on the phosphorus or potassium content. Half of the nitrogen should be applied as top dressing during the mid to late growth stages.
3. Compound fertilizers are generally not recommended for top dressing or foliar application. Even if the phosphate is fully soluble, it tends to remain near the soil surface, making it difficult for plant roots to absorb. Although some vegetable farmers in Guangdong use 45% (15-15-15) water-soluble compound fertilizer for foliar feeding, the efficiency is low, leading to waste and increased costs without significant benefits.
4. Low to medium concentration nitrogen-phosphorus binary compound fertilizers can be used as seed fertilizer, but care must be taken to separate seeds from fertilizer. Organic-inorganic compound fertilizers can be applied in large quantities as base fertilizers.
5. It is important to combine compound fertilizers with simple fertilizers. After applying compound fertilizer as a base, half of the nitrogen or a small amount of potash should be added as top dressing to balance nutrient proportions and meet the crop’s needs for high yield and quality.
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